An X-ray CT apparatus has an X-ray tube and an X-ray detector that are oppositely provided across the subject. The X-ray detector includes a plurality of channels of X-ray detection elements along the channel direction, which is perpendicular to the longitudinal direction of the couch (body axis direction).
Meanwhile, a multi-slice X-ray CT apparatus has a plurality of rows of X-ray detector elements not only along the channel direction, but also along the body axis direction of the subject. The X-ray detector of the multi-slice X-ray CT apparatus is configured as a two-dimensional detector for X-ray CT having M channels by N rows of X-ray detector elements.
The multi-slice X-ray CT apparatus executes a conventional scan using a FFS (Flying Focal Spot) method that enables the X-ray focal spot to fluctuate along the z-axis direction (body axis direction) around the original focal spot. Hereinafter, the FFS method for fluctuating the X-ray focal spot along the z-axis direction is referred to as the z-FFS method.
In the case of a conventional scan of a multi-slice X-ray CT apparatus using the z-FFS method, the focal spot is changed for each view. For example, the position of the focal spot of an odd-number-turn view is +σ, while the position of the focal spot of an even-number-turn view is −σ. The conventional scan causes the that sampling pitch of the data to be half, compared to the conventional scan without the z-FFS method. The flying focal spots ±σ are shown by following expression.
      ±    σ    =      ±                  d        ·        SOD                    4        ·                  (                      SDD            -            SOD                    )                    Here, the SOD (source to object distance) is defined as the distance between the focal spot of the X-ray tube and the rotational center (isocenter), the SDD (source to detector distance) is defined as the distance between the focal spot of the X-ray and the center of the X-ray detector, and d is defined as a pitch of the X-ray detector elements.